Your design is fundamentally flawed for your stated purpose. Do not implement it.
If you implement it, do not promise 5 nines availability.
If you implement it and promise 5 nines availability, have an EXCELLENT lawyer. You'll need one.
As ChrisS said, you need to **provide redundancy at every level*. At a bare minimum this means:
- Redundant upstream connections to the internet [provided by your ISP]
- Redundant UPS and generator-backed power [provided by your ISP]
- Redundant gateways/virtual routers [provided by your ISP]
- Redundant, physically separate paths from your ISP's core to your servers [provided by your ISP]
- Redundant firewalls and load balancing equipment [provided by YOU, though your ISP may provide it too]
- Redundant switching equipment [provided by YOU, though your ISP my provide it too]
- Redundant servers [provided by YOU, ideally with redundant subsystems: ECC RAM, RAID, dual power supplies].
Note that this list is not exhaustive (for example I've made no mention of cooling).
You also need to clearly define what your uptime is measured against - Five Nines 24x7x365 is nearly impossible to achieve. Five Nines excluding scheduled maintenance is much more reasonable. The specificity of your contract will mean the difference between making massive profit and being sued out of existence.
To directly answer your questions:
Can the 2 NICs on the same host be configured with different gateways?
Yes. This is called a multi-homed host and is one of several possible implementations. Actually making this work the way you want requires more than just throwing two "default" (0.0.0.0/0) routes into your routing table though.
Can NIC1 in the above diagram be configured with a backup gateway2, so that if gateway 1 crashes, NIC1 can use backup gateway2?
NIC1 has no connection to